For example your Cooper 2 walls. For that fact any large wall using a pattern or full size image such as the Carbon fiber backgrounds here. Want to try it. Just make a new image with a Black BG and add you Light golden cooper logo and gold border around it. Bet it won't be 2.5 megs.

Remember Jim they are large widescreen images. To an extent you have to expect a larger file size. Basically all colors without any patterns will greatly lower the size. Now I only used Copper as an example for this reply but it will apply to any wall large or small using a full pattern or even adding a pattern with a PS style. If you are using a full size pattern try making a much smaller PS pattern. Problem is you may not get it to look as you want it to.

Well, Jafo, it would seem that if I want the ultimate detail/quality, I need to upload what I made, regardless of "weight"?

Essentially yes.

Complicated 'pattern' images like carbon fiber/mesh patterns don't behave well if dithered down [or up] to suit screen resolutions other than which the original image was made in. It's why Windows generally has options to stretch/center/resize wallpapers so as to minimize image quality loss.

My current wallpaper is a 24bit bitmap at 2560x1440 [default monitor res] and runs to 10.5 meg. [11,063,296 bytes].... an in-game [FSX] screencap taken at that resolution with no alteration.